1. Field of the Invention
The invention relates to an electronic device, and particularly relates to a power supply device.
2. Description of Related Art
The power supply device has been broadly applied to the electronic technology field. Under the consideration of System on Chip (SoC), the power supply device is often as an electronic device embedded in a chip.
FIG. 1 is a schematic circuit diagram illustrating a conventional power supply device, which may serve as a power protection device for a USB port or serve as a power supply device of the USB port to charge a mobile device. FIG. 2 is a diagram illustrating a relation between an output voltage VO and an output current IO of the power supply device 100 shown in FIG. 1. Referring to FIGS. 1 and 2 together, the power supply device 100 includes a control circuit 102, transistors M1 and M2, and a resistor R2. Therein, the transistor M1 is coupled between a power voltage VDD and an output end OUT of the power supply device 100. The resistor R2 is coupled between the power voltage VDD and a node P. The transistor M2 is coupled between the node P and the output end OUT of the power supply device 100. Gate ends of the transistors M1 and M2 are coupled to the control circuit 102. An input end of the control circuit 102 is coupled to the node P, and another input end of the control circuit 102 receives a reference voltage VREF. The reference voltage VREF is related to an over-current-protection current value IOCP.
The transistor M1 provides a current IM1, and the transistor M2 provides a current IM2. An output current IO is a total of the current IM1 and the current IM2. Therein, the transistor M2 is a sampling transistor of the transistor M1. The resistor R2 is a sampling resistor. Since a size ratio between the transistor M1 and the transistor M2 is a fixed value, the transistor M2 may be used to sample the current IM1, and the control circuit 102 is further used to detect whether the output current IO exceeds the over-current-protection current value IOCP.
Specifically, the gate ends of the transistors M1 and M2 receive the same voltage signal, and source ends of the transistors M1 and M2 are coupled to each other. Thus, the currents IM1 and IM2 are related to the size ratio of the transistors M1 and M2 and drain-source voltage differences of the transistors M1 and M2. Therein, a voltage at the drain end of the transistor M1 is the power voltage VDD, and a voltage at the drain end of the transistor M2 is a sampling voltage VP at the node P. When the current IM2 flows through the resistor R2, the sampling voltage VP is generated at the node P. Thus, by detecting the sampling voltage VP, the current IM2 can be obtained. Then, based on the size ratio between the transistors M1 and M2 and drain-source voltage differences VDS1 and VDS2 thereof, the current IM1 can be obtained. Consequently, the output current IO is detected and known. In other words, variation of the output current IO can be indirectly detected through the sampling voltage VP. Since the sampling voltage VP is related to the output current IO, and the reference voltage VREF is related to the over-current-protection current value IOCP, the control circuit 102 may detect whether the current value of the output current IO is greater than the over-current-protection current value IOCP based on the sampling voltage VP and the reference voltage VREF.
When the current value of the output current IO is smaller than the over-current-protection current value IOCP, as shown in a region between points A and B in FIG. 2, the transistors M1 and M2 are operated in a linear region. At this time, the currents IM1 and IM2 are respectively related to the drain-source voltage difference VDS1 of the transistor M1 and the drain-source voltage difference VDS2 of the transistor M2. In addition, VDS1 is greater than VDS2.
Once the current value of the output current IO is equal to the over-current-protection current value IOCP, as shown at the point B in FIG. 2, the sampling voltage VP is equal to the reference voltage VREF. Once the current value of the output current IO is greater than the over-current-protection current value IOCP, the power supply device 100 enters the over-current-protection state. The control circuit 102 controls gate voltages of the transistors M1 and M2 according to the sampling voltage VP and the reference voltage VREF, and keeps the sampling voltage VP at the reference voltage VREF. At this time, based on loads, an operation region of the transistors M1 and M2 may be between points B and C or between points C and D. If a load impedance is smaller, the operation region of the transistors M1 and M2 may be transited from the linear region (between points B and C shown in FIG. 2) to a saturation region (i.e., a region between points C and D in FIG. 2), and a relatedness of the currents IM1 and IM2 and VDS1 and VDS2 is low. Since a current ratio when the transistors M1 and M2 are operated in the linear region is greater than a current ratio when the transistors M1 and M2 are operated in the saturation region, and a current when the transistor M2 is operated in the saturation region (the region between points C and D in FIG. 2) is substantially equivalent to a current when the transistor M2 is at an over-current point (point B shown in FIG. 2) (since the sampling voltage VP is kept at the reference voltage VREF in the regions between points B (including point B) and C and between points C and D in FIG. 2, the current flowing through the resistor R2 and the transistor M2 keeps the same), the current value of the output current IO (also called a short circuit current value ISC) of the power supply device 100 after entering the over-current-protection state is smaller than the over-current-protection value IOCP, as shown in FIG. 2.
Since the current value of the output current IO (i.e., the short circuit current value ISC) of the power supply device 100 after entering the over-current-protection state is lower than the over-current-protection value IOCP, the output voltage may be unable to be elevated to an effective level (e.g., a voltage level of 5V for a USB device). In addition, a charging current value provided to the mobile device may be lower, making charging slower. Although the short circuit current value ISC may be increased by increasing the over-current-protection current value IOCP, the accuracy of the short circuit current value ISC cannot be improved. This is because the current value of the output current IO is the total of current values of the currents IM1 and IM2, and there may be an inaccuracy in the size ratio between the transistors M1 and M2 and an inaccuracy in a resistance of the resistor R2, making the sampling voltage VP unable to correctly reflect the real current IM1 (or the real output current IO). Thus, the control circuit 102 is unable to accurately control the gate voltages of the transistors M1 and M2, and the accuracy of the short circuit value ISC is thus unable to be improved effectively.